Directional drilling usually involves down-hole drill bit driving drilling motors. The motors are usually powered by the drilling mud flowing down the drill string bore. The motors are usually of the positive displacement type. Such motors are currently limited in terms of drilling fluid pressure drop through the motor. Motor output power is proportional to the pressure drop. The acceptable pressure drop is usually much less than the available pressure that the surface pumps could supply. Drilling motors seldom deliver the maximum available horsepower to the drill bit.
The pressure drop through the motor is proportional to motor output torque, and modern drill bits are often capable of using more torque than drilling motors can provide. In many such cases, the common rotary practice can drill hole more economically without the motor. The motor, however, may be essential to well bore direction control.
If directional control is necessary, a down hole assembly often has to have steerable rotary apparatus or a motor with a bore deflecting ability. Either choice adds a technical and economical burden to the drilling activity.
There is a need for rotary drilling assembly that can accommodate a down hole motor. The motor is needed periodically to provide a hole direction influence. The motor needs to be capable of efficient activation and de-activation so that optimum rotary practice can be applied most of the drilling time.
If the drilling motor is used a small percentage of the drilling time, it can be made somewhat smaller than greater dependence upon the motor would allow. By reducing the motor size, the wall thickness of the motor housing can be thickened to accept greater rotary torque. Overall benefit is realized from the ability to readily switch from one mode of drilling to another.
When the motor is activated, it is advantageous to deflect the drilling assembly, ideally with the same down link command. That can be accomplished if the motor drives the deflection apparatus. While the motor runs, then, the deflection is effective. While the motor is disabled, then, the rotary system drives the drill bit in the straight hole drilling mode.
While the motor is activated, the progressing well bore needs directional control. Such control, in terms of orientation, can be provided by Measurement While Drilling (MWD) apparatus. The MWD apparatus, currently available, does not control drill string deflecting apparatus. To get the best use of drill string deflecting apparatus, there is a need for some form of down link command that is reliable and simple to exercise. In addition to down link command, there is a need to have some form or reaction feedback that indicates, in real time, that the command is being carried out.